Electrostatic analyzers with auxiliary focusing electrodes

ABSTRACT

An electrostatic analyzer of the energy spectrum of charged particles is provided comprising spaced apart analyzer plates adapted for operation in a velocity-focusing mode of operation and auxiliary electrode means adjoining an inner face of an analyzer plate. Circuit means apply a potential to each analyzer plate and a potential of opposite polarity to an associated auxiliary electrode means for establishing an electrostatic field between the plates which in a zone of influence of the auxiliary electrode means is modified to direct peripheral particle trajectories towards a focus substantially common with that of paraxial trajectories.

United States Patent David W. Turner l leadington, England Oct. 7, 1969Nov. 16, 197 l Perkin-Elmer Limited Beaconsfield, Buckinghamshire,England Inventor Appl. No. Filed Patented Assignee ELECTROSTATICANALYZERS WITH AUXILIARY FOCUSING ELECTRODES 8 Claims, 3 Drawing Figs.

US. Cl 250/4L9 ME, 250/49.5 P

Int. Cl H01 j 39/34 Field oi Search 250/4 1 .9 ME, 49.5 P

[56] References Cited UNITED STATES PATENTS 2,533,966 12/1950 Simmons,Jr. 250/4l.9 2,667,582 l/l954 Backus 250/41.9

Primary Examiner-William F Lindquist Att0rneyEdward R. Hyde, Jr.

ABSTRACT: An electrostatic analyzer of the energy spectrum of chargedparticles is provided comprising spaced apart analyzer plates adaptedfor operation in a velocity-focusing mode of operation and auxiliaryelectrode means adjoining an inner face of an analyzer plate. Circuitmeans apply a potential to each analyzer plate and a potential ofopposite polarity to an associated auxiliary electrode means forestablishing an electrostatic field between the plates which in a zoneof influence of the auxiliary electrpde means is modified to directperipheral particle trajectories towards a focus substantially commonwith that of paraxial trajectories.

PAIENIEnuuv 1s IHII SHEET 2 BF 2 ELECTROSTATHC ANALYZERS WITH-llAUXILIARY FOCUSING ELECTRODES This invention relates to electrostaticanalyzers for observing the energy spectrum of charged particles. Theinvention relates more particularly to Photoelectron Spectroscopy and toapparatus embodying electrostatic analyzers.

In photoelectron spectroscopy, a beam of photoelectrons is ejected froma sample gas within a target chamber by highenergy photons. The beamfans out over a small solid angle from a small aperture in the chamberinto a space between analyzer plates which are maintained at a selectedpotential difference. In passing through the analyzer the photoelectronsundergo velocity focusing, and in order to detect the different energiesthat may be present the potential difference between the analyzer platesis scanned over a selected range. The photoelectrons of successiveenergy values are thereby brought successively to a focus at a discreteexit aperture, behind which a detector such as a particle multiplier ispositioned.

Prior electrostatic analyzers have exhibited inherent aberrations whichdegrade the resolution of the observed spectra. Spherical aberration isparticularly prominent, and since it is brought about by the fact thatthe paraxial rays and the peripheral rays do not have a common focus, itcan be reduced by providing a smaller solid angle of radiation. This,however, cannot be carried far enough in practice to be fully effectivewithout at the same time reducing the detector signal to an extent thatcannot be tolerated. It is preferable to minimize spherical aberrationwhile retaining a reasonably large angle of acceptance.

It is an object ofthis invention to provide an improved energy analyzerarrangement for a spectrometer.

Another object of this invention is to provide an analyzer arrangementfor modifying the electrostatic field between the analyzer plates of aspectrometer in order that paraxial and peripheral rays of particles ofagiven energy will be brought to a substantially common focus.

In accordance with features of the present invention an electrostaticanalyzer of the energy spectrum of charged particles is providedcomprising spaced apart analyzer plates adapted for a velocity-focusingmode of operation and auxiliary electrode means adjoining an inner faceof an analyzer plate. Circuit means apply a potential to each analyzerplate and a potential of opposite polarity to an associated auxiliaryelectrode means for establishing an electrostatic field betweentheplates which in a zone of influence of the auxiliary electrode meansand is modified to direct the peripheral particle trajectories toward afocus substantially common with that of the paraxial trajectories.

The auxiliary electrode means in one embodiment comprises alongitudinally extending electrode recessed in an associated plate, thesurface of the electrode facing a median plane of the analyzer andshaped for causing the desired redirection effect without substantiallyinterfering with the continuity of the inner face of the plate. In analternative embodiment the auxiliary electrode means comprises aplurality of electrodes for enabling the shape of the modified field tobe controlled by the application of different potentials to theindividual electrodes.

Additionally, means are provided for reducing an adverse effect causedby fringing fields existing at an exit or entry region of anelectrostatic analyzer upon the resolution limit of the energy spectra.Eringing is reduced by arranging an arcuate conductive path each side ofthe exit aperture in the form, for example, of metal sheeting flaringout from the aperture towards the detector station at a constant orconveniently changing radius.

The invention will now be described, by way of example, in itsapplication to a cylindrical condenser configuration, of which thespherical condenser is a well-known variant in electrostatic analyzers.In the description which follows, reference shall be had to the attacheddiagrammatic drawings, wherein:

FIG. 1 is a plan view of a cylindrical electrostatic analyzer embodyingthe present invention; and

FIG. 2 is an electrical diagram of the analyzer energization.

FIG. 3 is a sectional view taken on the line 3--3 of FIG. i.

For a clearer understanding of its mode of operation and because we areparticularly concerned with the application of our invention tophotoelectron spectroscopy, we will assume that the electrostaticanalyzer to be described forms part of a photoelectron spectrometer,although the skilled in the art will readily appreciate that itsusefulness extends beyond the analysis of photoelectron energies.

FIG. ll shows a vacuum tank l with the top removed, and within it, atarget chamber 2 and an electron multiplier 3 adjoining the entry andthe exit, respectively, of the analyzer 4. The target chamber 2 is agenerally cylindrical tube in which a slit, say 1 cm. in length, isformed at the apex of two cuspshaped wall portions 2A. The slit definesthe area through which photoelectrons ejected from a sample gas in thetarget chamber 2 by photons emanating from a source not shown will fanout into a solid angle of radiation and represents in fact the entryaperture of the analyzer 4. The exit aperture of the analyzer 4 isdefined by juxtaposed antifringing cusp-shaped members 5. One of thesemay be moved through the micrometric adjustment means 6 reiative to theother, which is fixed. In this manner the spacing between the edges ofthe members at the apex of the cusp may be increased or decreased toalter the size of the exit aperture. Alternatively, a mechanism may beincluded for moving both members simultaneously and symmetrically inopposite directions. It is advantageous to include means for enablingtheslit to be moved in any direction and optionally means forcontrolling the cusps geometry.

The cylindrical plates forming the main electrodes of the analyzer 4 areshown at 4A and 4B. They are supported in spaced relation but insulatedfrom each other by a base plate 4C mounted on the bottom of tank I andby a top plate (not shown). From both the baseplate and the top plate amiddle cylindrical flange, such as 4D, extends towards the electrostaticaxis of the analyzer 4. The flanges terminateshort of said axis and thespacing between the two opposed cylindrical edges representsapproximately the height of the effective analyzer volume, of which thewidth is represented by the spacing between plates 4A and 4B. The objectof the flanges is to straighten the electrostatic field at the top andbottom of said effective volume. Plate 4A is slotted at 4E in adirection perpendicular to the plane of the paper and within the slotthere is supported a discrete electrically insulated auxiliary electrodemeans in the form of a narrow conductive strip 4F, to which anelectrical potential may be applied from outside the tank] throughinsulated terminal 7. An alternative to the slot 4E is a cavity formedin the plate 4A in which the strip 4F is supported with a spacing fromthe cavity wall. Slot 4G, strip 4H and insulated terminal 8 are thecounterparts of 4E, 4F and 7, respectively, in relation to plate 48.

In operation, a suitable potential difference scanned over apredetermined range is applied between plates 4A and 48 throughinsulated terminals 9 and 10 and a suitable and simultaneously scannedpotential difference of opposite sign is applied between auxiliaryelectrode means 4F and 4H. If the potentials are properly adjusted inrelation to the range of energies expected in the photoelectrons issuingfrom the target chamber 2, the well of potential created by eachauxiliary electrode means produces a bulge in the distribution of theelectrostatic field existing between the analyzer plates which tends toredirect the peripheral photoelectrons towards a focus substantiallycommon with that of the paraxial photoelectrons.

The effect of the two auxiliary electrode means should fall off rapidlyand ideally reduce to zero or nearly so at the median plane of theanalyzer. For this reason the width of the slot in which the auxiliaryelectrode means is located and the width of the active surface of theauxiliary electrode means, i.e., that facing the median plane, are bothmade small compared with the spacing between the plates 4A and 48 so asto achieve what might be termed a point source effect. This is themeaning intended in our reference to discrete" auxiliary electrodemeans.

In a particular embodiment of the invention a slot width of 3 mm. wasassociated with an electrode width of 1 mm. arld a plate spacing of 2cm. The active surface of the auxiliary electrode means may be given thesame curvature as that of the associated plate and may be mounted flush.

The internal surfaces of the analyzer likely to be struck by thephotoelectrons may be treated to give low emission of secondaryelectrons. We have used benzene soot and a colloidal graphite solutionwith equally good results.

Each auxiliary electrode means may comprise an array of two or moreelectrically individual elongated electrodes to each of which apotential of suitable value may be applied in relation to the potentialsapplied to each of the remaining electrodes. In using a multielectrodestructure with each analyzer plate, one would strive to achieve bygeometry and selective energization of electrodes a field distortingeffect which vanished to zero at the median plane and at any other pointintermediate between the electrode array and the median plane was of theright value to produce the right amount of redirection of the peripheralphotoelectrons to cause them to focus at substantially the same plane asthe paraxial photoelectrons.

The two potentials for the analyzer plates and the opposite potentialsfor the auxiliary electrode means are derived from a common supply. Thefour potentials are swept together in a manner mechanically slaved tothe chart recorder. In FIG. 2, block 30 represents a DC power supply(e.g., 50 volt supply), R1 and R6 are ganged rheostats for setting thescan datum, VR3 is the slaving potentiometer driven by thechart-advancing mechanism of the recorder, VRI and VR2 arepotentiometers for setting the ratio between plate potential andauxiliary electrode means potential, R7 and R8 are resistors ofconvenient value for the energization of the plates, and VR is apotentiometer for balancing to ground. The analyzer plates are indicatedat 31 and 32 and the respective auxiliary electrode means at 33 and 34.Finally R25 are scale expansion resistors. In operation, R1 and 6 areadjusted in accordance with the requirements of the analysis to becarried out, but VRl, VR2 and VR3 are essentially preset controls.

Representative values for the components shown of FIG. 2 are as follows:

Rldt R6 1,0001] The FIG. 2 arrangement is slightly modified to giveindependent adjustment of the potentials to ground extended toelectrodes 33 and 34 while still ensuring that they are swept as theanalyzer plate potentials are swept. This modification VR and VR in FIG.2 are replaced with fixed resistors of equal and suitable value andconnecting two potentiometers in series across the chain comprising R-,,VR;,' and R the sliders of the two potentiometers being routed to theauxiliary electrodes in the manner shown for VR and VR in FIG. 2.

Whatever the potential existing across the chain comprising VR,, R VR Rand VR for any setting of the ganged potentiometers VR and VR there willbe a given ratio of potential applied to an auxiliary electrode to thatapplied to the analyzing plate.

Potentiometers VR balances when its slider is exactly midway and VR andVR are electrically and mechanically identical. Thus with their slidersganged, when the slider of VR is at the bottom of the VR, winding andthe slider of VR is at the top of the VR winding then plate 32 willreceive a positive potential to ground which is the potential dropacross R-, and half of VR;,' in series and the same positive potentialwill be applied to the auxiliary electrode 33. Similarly, plate 31 willreceive a negative potential to ground equal in value to the potentialdrop across R and the other half of VR;,. This negative potential willalso be extended to auxiliary electrode 34. The ratio of an auxiliaryelectrode potential to the potential of the adjoining plate willtherefore be l:l. Now, as shown on FIG. 2, the sliders of VR, and VR,are ganged so that they move in opposite directions. If we increase thesetting of the ganged sliders, the positive potential received byauxiliary electrode 33 will be augmented by the potential drop appearingbetween the slider of VR, and the resistor R At the same time thenegative potential received by auxiliary electrode 34 will be augmentedby the potential drop appearing between the slider of VR and theresistor R Therefore the ratio of the auxiliary electrode potential tothe plate potential is clearly greater than l:l.

If we turn the gang to its maximum setting, the potential increasereceived by the auxiliary electrodes will be the total potential dropacross the respective potentiometer windings. At this setting, the ratioof auxiliary electrode potential to plate potential is found by dividingthe series resistance of VR, plus R, plus half of VR;, by the seriesresistance of R, plus half of VR:,. Substituting the values given in thespecification we have 500+47+50/47+50=597/97 which gives a ratio ofapproximately 6:1.

The ratio of auxiliary electrode potential to plate potential may beadjusted therefore in the range 1:1 to 611. Since the sweep potentialprovided by the slave potentiometer VR; is applied across the ends ofthe chain comprising VR R VR R and VR it follows that any ratio selectedin the range 1:1 and 6:1 is maintained during a potential sweep of theanalyzer plates.

It has been found that in so far as the potentials applied to the twoauxiliary electrodes control the spherical aberrations of the analyzer,the fine setting of these potentials may be used with advantage for thefinal adjustment of energy concentration at the exit slit of theanalyzer.

In photoelectron spectroscopy, the chamber 2, the photomultiplier 3 andthe analyzer 4 are conveniently arranged for continuous vacuumoperation, hence the need for vacuum tank I intended to be connected toa vacuum pump, not shown.

In an alternative embodiment to the analysis of the electron energyspectrum by scanning the voltage applied to the analyzer plates in themanner referred to, the plates are maintained at a suitable fixedpotential and the voltage applied to an apertured electrode, placedbetween the effective electron source which in the embodiment describedis the slit at the apex of the two cusp-shaped wall portions 2A and theentry to the analyzer is scanned. The voltage is alternatively anaccelerating or a decelerating voltage and the aperture through whichthe electrons are either accelerated or decelerated is shaped forelectron-optical purposes.

An improved electrostatic analyzer arrangement for reducing aberrationsin an electron spectrometer has been described. While this particularembodiment of the invention has been illustrated and described, it willbe understood that various modifications may be made therein withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

What is claimed is:

1. An electrostatic analyzer of the energy spectrum of charged particlescomprising a pair of spaced apart analyzer plates adapted for thevelocity-focusing mode of operation and auxiliary electrode meansadjacent the inner face of each of said plates, means to electricallyinsulate each auxiliary electrode from its adjacent analyzer plate,means for applying a potential of first polarity to each plate and apotential of opposite polarity to the associated auxiliary electrodemeans for modifying an electrostatic field between the plates in thezone of influence of the auxiliary electrode means to direct peripheralparticle trajectories towards a focus substantially common with that ofparaxial trajectories.

2. An analyzer as claimed in claim 1, wherein the auxiliary electrodemeans comprises an elongated electrode means recessed in said inner faceat a location intermediate between the entry and the exit of theanalyzer.

3. An analyzer as claimed in claim 2, wherein a surface of iliaryelectrode means extends in length so as to span the two the elongatedelectrode means facing the median plane of the flanges. analyzer issubstantially flush with said inner face. 6, A al a laimed i n claim 4,wherein antifringing 4. The analyzer of claim 1 wherein said platesdefine a cylinmeans is provided at the exit of the analyzer.

drica] Fondfmser and the length the auxillaty electrode 5 7. Ananalyzeras claimed in claim 6, wherein said antifring- 3: 2:gzt'zg gigfggf gg around much the ing means is a pair ofcus p-shaped conductors. i

5. An analyzer as claimed in claim 4, comprising antifring- P as claimedm Clam 4 where, amf'mgmg ing cylindrical flanges lying in the medianplane symmetrically means provlded at the entry oflhe analyzer spacedfrom the electrostatic axis of the analyzer and the aux- 10

1. An electrostatic analyzer of the energy spectrum of charged particlescomprising a pair of spaced apart analyzer plates adapted for thevelocity-focusing mode of operation and auxiliary electrode meansadjacent the inner face of each of said plates, means to electricallyinsulate each auxiliary electrode from its adjacent analyzer plate,means for applying a potential of first polarity to each plate and apotential of opposite polarity to the associated auxiliary electrodemeans for modifying an electrostatic field between the plates in thezone of influence of the auxiliary electrode means to direct peripheralparticle trajectories towards a focus substantially common with that ofparaxial trajectories.
 2. An analyzer as claimed in claim 1, wherein theauxiliary electrode means comprises an elongated electrode meansrecessed in said inner face at a location intermediate between the entryand the exit of the analyzer.
 3. An analyzer as claimed in claim 2,wherein a surface of the elongated electrode means facing the medianplane of the analyzer is substantially flush with said inner face. 4.The analyzer of claim 1 wherein said plates define a cylindricalcondenser and the length of the auxiliary electrode means is orientedparallel to the axis around which the condenser is geometricallygenerated.
 5. An analyzer as claimed in claim 4, comprising antifringingcylindrical flanges lying in the median plane symmetrically spaced fromthe electrostatic axis of the analyzer and the auxiliary electrode meansextends in length so as to span the two flanges.
 6. An analyzer asclaimed in claim 4, wherein antifringing means is provided at the exitof the analyzer.
 7. An analyzer as claimed in claim 6, wherein saidantifringing means is a pair of cusp-shaped conductors.
 8. An analyzeras claimed in claim 4 wherein antifringing means is provided at theentry of the analyzer.